The present invention relates generally to safety belt restraint systems for motor vehicles and, more particularly, to a belt tensioning mechanism operable to automatically retract a belt buckle for tensioning the safety belt against the seat occupant in response to the vehicle being subjected to a deceleration force exceeding a predetermined critical value.
Modernly, virtually all motor vehicles are equipped with some type of passenger restraint system for physically restraining the seat occupant when the vehicle is subjected to high rates of deceleration which may occur, for example, during heavy braking or a collision. As is known, the most common type of passenger restraint system is a safety belt system having at least one of a lap belt and a shoulder belt, or a combination thereof, that can be withdrawn from a belt retractor for latched engagement with an anchored belt buckle. One type of belt retractor, commonly referred to as an "emergency locking retractor" (ELR), permits the seat belt to be freely withdrawn in response to movement of the seat occupant during normal operating circumstances, thereby providing increased comfort for the seat occupant. However, such emergency locking retractors commonly include an inertia-sensitive locking mechanism for automatically inhibiting withdrawal of the safety belt when the motor vehicle is subjected to deceleration forces exceeding a predetermined critical level. In addition, many belt retractors are also equipped with a tension relief device or so-called "comfort" mechanism which prevents the retractor from constantly exerting a tensioning force on the belt webbing which tends to be uncomfortable and annoying to the seat occupant. Typically, the "slack" generated by the comfort mechanism can be cancelled by withdrawing a relatively short length of the belt from the retractor.
While conventional belt retractors generally perform satisfactorily, the existence of "slack" in the seat belt (i.e., such as when the seat occupant is sitting forward in the seat) may permit the seat occupant to be thrown forward and collide with on-board structures when such an excessive deceleration force is exerted on the motor vehicle. To prevent such secondary impacts, the protection which is afforded by conventional safety belt restraint systems can be further enhanced by the use of a belt tensioning mechanism. In general, belt tensioning mechanisms function to automatically remove slack from the safety belt immediately after detection of such a deceleration force such that the seat occupant is physically restrained within the seat before being thrown forward due to the inertia of the collision.
Many conventional belt tensioning mechanisms are incorporated into the belt retractor and operate to retract the safety belt into the retractor upon detection of the deceleration force. Alternatively, it is also known to use a separate belt tensioning mechanism for causing retractive movement of the belt buckle such that existing "slack" in the safety belt is removed. For example, commonly owned U.S. Pat. No. 5,211,423 discloses a belt tensioning mechanism that is responsive to vehicle deceleration levels above a predetermined critical value for retracting the belt buckle so as to apply a "tensioning" load on the safety belt, thereby minimizing forward excursions of the seat occupant. More specifically, this system employs an inertia-sensitive triggering arrangement for actuating a preloaded drive mechanism in response to detection of the excessive deceleration force. In addition, a ratchet-type locking mechanism is also provided for maintaining the belt buckle in the retracted condition to inhibit subsequent forward movement of the belt buckle due to the loading exerted thereon by the seat occupant due to the inertia of the collision.
While such conventional buckle-type belt tensioning mechanisms are satisfactory for their intended purpose, a need exists to continuously develop new and improved belt tensioning mechanisms that enhance the overall effectiveness of safety belt restraint systems by decreasing the reaction time required to sense an excessive deceleration condition and retract the belt buckle.